Current balancing circuit

Abstract

The invention provides a current balancing circuit for powering a first load with a negative dynamic resistance and a second load with an essentially different negative dynamic resistance, and a transformer with such turns ratio that at least at an operating frequency impedances of the first and second windings complement the resistances of the first and the second loads, respectively, to a common value.

1 . Load feed circuit comprising:
 a first load with a first negative dynamic resistance;   a second load with a second negative dynamic resistance, the second load essentially different from the first load;   a transformer comprising a first transformer winding and a second transformer winding coupled magnetically, the first transformer winding being connected in series with the first load, and the second transformer winding being connected in series with the second load;   
     wherein: 
     the numbers of turns of the first transformer winding and the second transformer winding are selected with such a difference that at least in operation at a predetermined operating frequency an unbalance between the current through the first transformer winding and the current through the second transformer winding is compensated for. 
   
   
       2 . Circuit according to  claim 1 , wherein the first load comprises a fluorescent lamp. 
   
   
       3 . Circuit according to  claim 2 , wherein the second load comprises a series connection of a number of fluorescent lamps different from a number of fluorescent lamps of the first load. 
   
   
       4 . Circuit according to  claim 1 , wherein a transformer winding connected in series with the load having the largest load negative dynamic resistance is selected to have the lowest number of turns. 
   
   
       5 . Circuit according to  claim 1 , wherein the first and the second loads are powered at an operating frequency between 20 kHz and 500 kHz. 
   
   
       6 . Circuit according to  claim 1 , wherein a load current is pulse width modulated. 
   
   
       7 . Circuit according to  claim 6 , wherein the pulse width modulation has a pulse frequency between 45 and 500 Hz. 
   
   
       8 . Circuit according to  claim 1 , wherein a coupling factor of the transformer is about 1. 
   
   
       9 . Circuit according to  claim 1 , wherein the transformer has a low self-inductance. 
   
   
       10 . Circuit according to  claim 1 , wherein an impedance of the first winding and an impedance of the second winding are selected to have an average value that equals an average value of a resistance of the first load and a resistance of the second load. 
   
   
       11 . Circuit according to  claim 1 , wherein a difference between an inductance of the first winding and an inductance of the second winding are selected such that said difference divided by the average of an inductance of the first winding and an inductance of the second winding equals half the value of the difference between a resistance of the first load and a resistance of the second load, divided by the average value of the resistance of the first load and the resistance of the second load. 
   
   
       12 . Transformer configured for use in a circuit according to  claim 1 . 
   
   
       13 . LCD display device, comprising a load feed circuit according to  claim 1 . 
   
   
       14 . LCD display device according to  claim 13 , wherein the first load and the second load each comprise at least one fluorescent lamp for backlighting the display. 
   
   
       15 . Method for balancing a current in a load feed circuit, comprising the steps of:
 providing a first load with a first negative dynamic resistance;   providing a second load with a second negative dynamic resistance;   providing a transformer having a first transformer winding and a second transformer winding magnetically coupled to each other;   connecting the first transformer winding in series with the first load;   connecting the second transformer winding in series with the second load;   selecting the turns ratio of the transformer such that at least in operation at a predetermined operating frequency an unbalance between the current through the first transformer winding and the current through the second transformer winding is compensated for.